Quickly and accurately estimating the location of things within a geographic area can be very useful. For example, information regarding the location of people or items can be used to speed up emergency response times, track movement of items and people, and link consumers to nearby businesses. Most approaches rely on a process called trilateration. Trilateration uses geometry to estimate the position of an object using distances traveled by different positioning signals (also referred to as “ranging” signals) that are transmitted from three or more transmitters to receivers that are co-located with the object to be located.
Various networks of transmitters have been used to transmit positioning signals. For example, orbiting satellites in the Global Positioning Satellite (GPS) system transmit positioning signals. Each GPS satellite transmits a positioning signal on which a coarse/acquisition code is modulated. The positioning signal is received by a GPS receiver. The GPS receiver identifies the time the positioning signal was transmitted by the satellite. The receiver also determines a relative time of arrival based on an internal clock in the receiver. Once the transmission time and the relative reception times of the positioning signal are known, the receiver uses measurements from at least three satellites to solve a set of simultaneous equations to determine the position and relative clock offset for the receiver.
Unfortunately, GPS signals are very faint, which means that the signals require integration over a very long time for a receiver to acquire a GPS signal and to demodulate enough information from the signal to determine the range measurements necessary to determine the location of the receiver. In many cases, the range measurements are not as accurate as may be desired due to the fact that the signals may take indirect paths from the GPS satellites to the receiver. Acquiring enough information to compute an estimated range measurement associated with a direct path from each GPS satellite to the receiver may take additional time and processing power.
Receiving weak GPS signals in urban environments poses additional problems. Weak GPS signals often cannot reach receivers through buildings. In addition, in such urban environments, GPS signals are more likely to take multiple paths by reflecting off buildings. As noted above, such “multipathing” disrupts a receiver's ability to accurately estimate a range measurement between the receiver and the satellite. One way to address the challenges of determining the position location of a device in an urban environment is to use terrestrial transmitter systems. Such terrestrial transmitter systems provide stronger signals. Furthermore, terrestrial transmitter systems can include transmitters at different locations within the urban environment that reduce the number of instances when there is no direct path between the satellite and the receiver.
Examples of terrestrial transmitter systems that transmit positioning signals are described in U.S. Pat. No. 8,130,141 (the “'141 patent”). In at least one embodiment of the '141 patent, each terrestrial transmitter uses a GPS-like channel to transmit a precisely-timed positioning signal. The receiver computes its location by processing the positioning signals from three or more terrestrial transmitters, similar to the way in which the receiver would process GPS positioning signals from three or more GPS satellites. Since most (if not all) receivers understand how to process GPS signaling, the signals transmitted by such a terrestrial transmitter system can be used by existing receivers with minimal (or no) modifications to those receivers.
Even though terrestrial transmitter systems provide a more-reliable positioning service than GPS, such terrestrial transmitter systems may require a substantial amount of additional infrastructure to transmit the terrestrial positioning signals. Therefore, it would be desirable to reduce the need for such additional terrestrial hardware.
In addition to using terrestrial positioning signals transmitted from stations dedicated to sending such positioning signals, cellular telephones are capable of performing ranging measurements, such as observed time difference of arrival (OTDOA) measurements on cellular telephone signals, such as LTE (Long-Term Evolution) signals. Such signals are transmitted with waveforms organized in resource blocks. Each resource block consists of 7×12 resource elements in the case of normal cyclic prefix. In one embodiment in which an extended cyclic prefix is used, a Physical Resource Block (PRB) only has 6 symbols. A resource element represents the allocation of one symbol in time to one OFDM (Orthogonal Frequency Division Multiplexing) sub-carrier in frequency. A typical LTE waveform is transmitted at a frequency that is different from the frequency of dedicated terrestrial positioning systems, such as systems that conform to the well-known M-LMS industry standard. Typical LTE waveforms used to communicate positioning information comprise 50 resource blocks transmitted over a period of 6 ms. Accordingly, when determining the time difference of arrival of such LTE signals, the received signal is integrated over a period of no more than 6 ms. Restricting the positioning signals to 6 ms reduces the amount of bandwidth consumed by the positioning signals.
While it is advantageous to use LTE signals for ranging due to the ubiquitous nature of the LTE, there are several deficiencies with using the LTE signals for ranging. For one, it would be advantageous to be able to integrate over longer times. In addition, it would be advantageous to have waveforms that conform with a format optimized for position location rather than to a cellular telephone format, such as the signals that are transmitted by M-LMS (Multilateration and Location Monitoring Service) systems.